Dealing with risk is always about weighing the risk of the activity against other factors. This is true with any type of insurance. Sure, you can protect yourself from just about any risk these days with an appropriate insurance policy but does the cost really justify the protection given the statistics of coming to harm from said risk?

While you are correct that wearing a helmet might add some benefit in a crash scenario, the statistics show that accidents leading to injury or death are low enough that it is conceivable many pilots will go their entire flying life without ever experiencing one. Therefore, knowing that the odds are low, the risk for many pilots (in regards to non-aerobatic GA flight) does not outweigh the inconvenience and even added danger (restricted peripheral vision comes to mind) wearing a helmet poses.

It is admirable that you are concerned about the risk but to do this justice you really need to understand the statistics of the activity, too. Consider this: If you are thinking about your head you should also wear a helmet while operating or riding in an automobile as well, since head injuries are a risk there, too. Do you do that?

edit: My understanding of helicopter pilots is that those who wear helmets wear them for reasons other than head protection, namely built-in headsets and the ability to mount night vision goggles and other flight enhancement gear to the helmets. Race car operators do so because the risk of accident is so much higher than other activities.